If you see disabled or not compatible extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar ("Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

If this hasn't helped then also delete the addons.sqlite file.

Note that if you delete these files then you will have to confirm installing all extensions added by external, third-party, software.

If you see disabled or not compatible extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar ("Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

If this hasn't helped then also delete the addons.sqlite file.

Note that if you delete these files then you will have to confirm installing all extensions added by external, third-party, software.

It is possible that there is a problem with the file(s) that store the extensions registry.
Delete the extensions.* files (e.g. extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.
*https://support.mozilla.org/kb/Profiles
New files will be created when required.
See "Corrupt extension files":
*http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
*https://support.mozilla.org/kb/Unable+to+install+add-ons
If you see disabled or not compatible extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar ("Find Updates" button in older Firefox versions) to check if there is a compatibility update available.
If this hasn't helped then also delete the addons.sqlite file.
Note that if you delete these files then you will have to confirm installing all extensions added by external, third-party, software.